Support for legacy source names

[philsmt]

As part of the data reduction effort, files from offline corrections will end up in the same folder as raw data. Attempting to open this folder as a single data collection would load files covering the trains for the same source names, which EXtra-data prevents. Therefore we aim to rename source names for data resulting from offline corrections.

To make the transition period easier, we can insert an h5py.SoftLink under the old source name. This allows transparent access under the original name, while we can make EXtra-data aware of the link for the purpose of duplication checks. Additionally, access via the legacy name should return the same SourceData as the proper name, rather than one for the legacy name.